PROJECT SUMMARY The testis and epididymis mark the initial portion of the male genital tract (MGT) and are impermeable to most hydrophilic compounds due to tight junctions. This barrier ? the blood- testes barrier (BTB) ? can act as an obstacle for therapies requiring entry into these tissues for full effect; for example contributing to testicular relapse in acute lymphoblastic leukemia (ALL) or acting as a sanctuary site for HIV infection. However, these epithelial barriers are known to express alternative xenobiotic transporters that may be utilized to allow select agents to gain access to the MGT and reduce the frequency of testicular relapse or HIV transmission. Rather than studying why drugs fail to cross the BTB, the major focus of our studies is to characterize the endogenous transport processes that could allow drugs to access the MGT. We hypothesize that the penetration of MGT relevant drugs into the BTB occurs through transport processes that can be modeled computationally, and that therapeutic concentration of these drugs is greatly affected by epididymal transporters and water reabsorption. Our grant focuses on 3 aims: Aim 1: Establish the transepithelial transport mechanisms by which MGT relevant drugs can cross the BTB and develop a predictive model for rational therapeutic design. Aim 2: Determine the impact of epididymal water reabsorption on drug concentrations within the MGT in vivo. Aim 3: Determine whether nucleoside transport is supported in the epididymis and the impact on nucleoside-based drug disposition. Understanding these pathways will be foundational for the development of drugs, such as antiviral, chemotherapy, contraception, and fertility agents, that require access to the MGT in order to achieve full therapeutic effect.